The danger of electrocution or of serious shock to an individual coming in contact with an appliance surface having an electrical potential differing from ground potential when the individual was also touching a grounded surface created the need for ground fault circuit interrupters. Ground fault circuit interrupters cut off the supply of electric current to a "hot" surface immediately upon flow of electric current to ground through other than the normal return path of the circuit in which the appliance is connected. Early ground fault circuit interrupters continuously measured electrical potential to ground and cut off power to the monitored appliance in response to a predetermined change in potential. These early devices were inherently faulty in that it was difficult to obtain a true ground point from which to measure electrical potential and variations in ground impedance adversely affected their operation.
Improved ground fault circuit interrupters were later developed. These monitored the difference between the electrical currents flowing into and out of the power line to which they were connected. Ideally, the supply and return currents are equal and their difference has a magnitude of zero. Should an external electrical path from the phase or hot side of the supply line to ground be completed, the current at the grounded neutral terminal of the supply line decreases relative to the current at the phase terminal of the line. The improved ground fault circuit interrupters employed a differential transformer which generated a pulse upon the occurrence of an imbalance between the phase and neutral currents and this pulse was applied directly to a solenoid causing the solenoid to actuate thereby opening a set of contacts between the power line and appliance or load. In order to function such ground fault circuit interrupters required very large transformers, too bulky to permit mounting of the ground fault circuit interrupter in a standard wall receptacle and highly sensitive solenoids which like the transformers were very expensive.
Prior art ground fault circuit interrupters were also prone to tripping due to noise on the power line which created momentary imbalance between the phase and neutral currents. Other problems associated with prior art ground fault circuit interrupters included an inability to cut off power if inadvertently wired into a circuit in reverse, that is, with the load terminals connected to the power line and power line terminals connected to the load. In these devices when the ground fault circuit interrupter was connected in reverse, not only was the power continuously supplied to the load irrespective of actuation of the interrupter, but also in cases where the opening of the contacts was accomplished by an electromagnet, current would continue to flow in the electromagnet after the interrupter was actuated and the coil, normally designed only for intermittent duty, would burn out.
Another problem found in the prior art ground fault circuit interrupters was attributable to the use of contact reset mechanisms which permitted a user to open and reset the contacts at will. To insure utmost reliability in a ground fault circuit interrupter, it is desirable that the contact tripping mechanism be subject to limited duty, i.e., only when a ground fault occurs. Prior art ground fault circuit interrupters permitted a user to open the contacts manually in the absence of a ground fault thereby offering the temptation for the user to use the reset mechanism as a switch for turning an appliance on or off from the power outlet. Such use often resulted in the deterioration of the contacts and compromised the reliability of the ground fault circuit interrupter.